1. Field of the Invention
The present invention relates to infusion of medication into patient and, more particularly, to a system and method for detecting occlusion in a medication infusion system using pulsewise pressure signals.
2. Description of the Related Art
Occlusion of a fluid path is a complication where either the delivery to or withdrawal of fluid from a patient is partially or completely restricted. These include devices for SC, IM, ID and intravenous (IV) delivery, access and sampling. For example, in an ambulatory insulin infusion system, both basal rate and bolus delivery of a medication fluid to a patient is typically provided by delivery of micro-boluses or fluid pulses through a fluid path (e.g., a tube) to generate the composite target total delivery volume and rate, and delivered to the patient via an infusion set. Generally, the boluses during the basal infusion are periodically delivered in short pulses over a regular interval (such as a period of 3 minutes) via a servo motor that actuates a piston. The actuated piston moves and biases the fluid in a fluid reservoir, thereby decreasing volume in the fluid reservoir and causing a controlled amount of medication fluid to flow from the fluid reservoir and into the fluid path. The infusion set receives the fluid flow and communicates the fluid into the patient. After delivering the bolus, the system waits for the period to expire to initiate a next delivery of medication. During delivery of higher volumes (such as for post-prandial meal boluses), the size of the small individual pulses may be increased and/or the time interval decreased to provide a greater total fluid volume and increased delivery rate.
As the fluid flows through the tube toward the infusion set, the induced pressure in the infusion system decays as a result of losses due to mechanical forces (e.g., static and dynamic friction, and so on). Further, other external or internal factors may further impede the flow of fluid. A partial kink in the tubing would reduce cross-sectional area in the fluid path, thereby reducing the rate of fluid able to traverse the fluid path and increasing pressure in the fluid path. The fluid path may be impeded by other factors such as crystal formation in the fluid, the presence of gaseous bubbles, impurities or other particles, backpressure from tissues in the patient, physical movement of the patient, movement of the fluid path, non-compliance of elastomeric components in the fluid path, and so on. When the fluid path is disrupted by any internal or external reason, the fluid path may experience a complete or partial occlusion that affects delivery of the medication fluid to the patient.
The flow of the medication fluid in the fluid path is currently detected by measuring the force applied to the piston during piston actuation as described above. However, the force applied to the piston can reflect static and dynamic friction forces associated with the piston mechanism in addition to pressure in the fluid path. Thus, the force applied to the piston represents the combined static friction, dynamic friction, other mechanical forces in addition to fluid pressure. The fluid pressure may in fact be a relatively small component of the overall force applied to reservoir piston, and accordingly piston force is not necessarily directly correlated to the pressure in the fluid path at the location of medication delivery. As a result, sensitivity is limited in these types of systems since the static and dynamic friction forces within the fluid reservoir dominate below approximately 2 psi. It may take multiple piston movements to determine that there is an occlusion occurring in the fluid path that is presently affecting medication delivery. Further, in the event that the pressure of the fluid reservoir is low, the static and dynamic friction forces associated with piston movement may be larger than the force required to move the liquid, thereby causing the pressure measurements during piston movement to be inaccurate and prevent detection of occlusions.
Occlusion events are responsible for premature removal of 5-15% of vascular access devices such as peripheral intra venous catheters (PIVCs) that are used both for patient fluid sampling and medication delivery. Evidence suggests that timed or scheduled removal of PIVC catheters without cause may not benefit patients and may add cost to healthcare treatment. In a PIVC catheter, occlusion may be a result of mechanical phenomena such as kinking or impingement of the catheter tip against the intima, biochemical effects such as precipitation of the infusate, and thrombus formation. In particular, thrombus aggregation in a catheter may cause an occlusion event that leads to other complications such as phlebitis. In a PIVC catheter, blood can enter the catheter during events such as placement of the catheter, as a result of pressure changes from movements of the catheter or associated tubing, during checks performed by medical staff, as a result of improper or incomplete flushing of the catheter, or via blood sampling. Each blood exposure event in the catheter can lead to build up of thrombus within or around a catheter to form a clot that reduces the diameter of the flow path. Consequently, more pressure is needed to deliver the same amount of fluid at a given rate with potentially dangerous consequences for the patient.
In conventional systems an occlusion in the fluid path may be detected too slowly or not at all in some circumstances, with potentially dangerous consequences for the patient. For instance, if an undetected occlusion occurs during insulin infusion, the patient may not receive a necessary amount of medication to prevent a potentially dangerous hyperglycemic event. Because the delivery of the medication fluid may be vital in delivery of medical service, there is a need for rapid detection of occlusions in medication delivery systems.